This invention relates generally to air cushion vehicles and more specifically to the fabrication of electrically conductive and semiconductive flexible skirts that are used to form the primary or basic air cushion peripheral sealing means.
Whether the air cushion vehicle is of the fully skirted type or of a sidewall type in which the air cushion containment is effected by a combination of rigid side walls and bow and stern flexible skirts, the skirt materials have frequently been fabricated of a very durable fabric, which is preferably coated or impregnated on both sides with a rubber material to provide added strength and abrasion resistance. These air cushion vehicles, more particularly when operating in cold, dry environments, are prone to a large build up of electrostatic energy, which if not timely dissipated or bled off via suitable grounding paths or circuit means, will build up to varying significant potentials which cause not only incessant nuisance arcing shocks to the crew personnel, but which also cause extensive very serious high voltage discharging shocks which impart significant damage especially to the vessel's various electrical and electronic equipment.
Static electricity is caused by concentrations of electrons in one location, and the positively charged ions in another. When the voltage potential between the two locations is sufficiently large, the current flows from one location to the other, through the air. This is normally called "arcing". The distance that the arc can jump is determined by the voltage potential. The larger the distance, the larger the charge has to be in order to jump. Conversely, at a given voltage potential, if the distance between the points decreases enough, an arc will pass.
One of the causes of static electricity is the stripping of electrons by the motion of the air. It is theorized that this situation occurs on an air cushion vehicle due at least partially to the effect of the rotating machinery. In the case of the exemplary FIG. 1 vehicle, this would be the propellers and the lift fans. Since the airflow of both of these are obstructed by the engine compartment and the lift fan solutes, the structure may also be charged by the passing air. If this is so, a charge may also be generated just by the relative motion of the craft at high airspeeds which often approaches 70 mph.
The amount of charge which is generated is unknown. It is known that static electricity buildup is a function of the temperature and humidity. A decrease in either of these will increase the rapidity of static electric buildup. This is why such vessels operating in the cold arctic environment experience the static electric problems.
While static electricity potentials are commonly dissipated from heavy highway transporting trucks via the use of trailing chains or other conductive strap members beneath the truck bodies, similar attempts with the air cushion vehicles were unsuccessful. In this regard, in addition to attaching grounding straps to various equipment and personnel, pieces of tin foil were hung in the air and a metal chain was hung overboard to drag along the ground. The only method which seemed to have any effect was that of the dragging chain. However, this did not dissipate enough of the electrical charge to stop equipment damage and personnel nuisance shocks. Moreover, such elongated straps and long chains were found to be susceptible to detrimental snagging on foliage and other ground oriented objects or obstacles.
In addition to shocks to the fingers when touching metal objects, the arcing was so bad that the headsets would arc to the crew's outer ears. This is most distracting when the crew members are trying to see through the blowing snow and darkness, and maneuver the craft. The biggest problem experienced was the discharge of static electricity causing damage to the electronic equipment. Printed circuit boards are quite damage sensitive to current shocks which pass during the discharge. These boards are not usually repairable at the local maintenance shops and have to be replaced, or the whole unit has to be sent back to the manufacturer to be repaired. The result is either a large inventory of repair parts to keep the unit serviceable, requiring additional storage space in the arctic which is a remote and logistically difficult area to support, or a large amount of downtime of equipment. Both of these options result in costly operations.